The Obama administration Tuesday praised law enforcement officials who responded to and dismantled a car bomb in New York City last weekend and arrested a suspect late Monday. But the fact remained that Faisal Shahzad was allegedly able to train with terrorists in Pakistan, return to the United States to assemble a car bomb in Connecticut and park it in Times Square without anyone in the nation's vast counterterrorism apparatus knowing anything about it.

Senior administration officials cited two instances in which the system could have worked more effectively, both of them after the bomb was found. On Monday night, sometime between the FBI's discovery of Shahzad's identity and whereabouts and his removal from an Emirates airline jet that was about to depart from John F. Kennedy International Airport, agents "lost him," one official said.

"It does beg the question why he wasn't apprehended before arriving at the airport or boarding the plane," the official said, speaking on the condition of anonymity after the administration decreed that only the departments of Justice and Homeland Security could comment for the record.

Officials also pointed to Emirates' failure to update its no-fly list in response to federal bulletins Monday afternoon, allowing Shahzad, who arrived at the airport at 7:35 p.m. after booking his flight from his car on the way, to board the 11 p.m. flight.

Another senior official acknowledged that the process is not foolproof. "We have a system that is built with redundancy and that is agile, increasingly so," he said. "So while it's our job to worry -- and to act on those worries -- we also feel like we have a system that's been improved over time."

But some Republicans, noting that the attempt was not prevented, criticized the administration for its actions after the plot was uncovered, including reading Miranda rights to Shahzad hours after his arrest. Administration officials said he was initially questioned without those rights, under an exemption that allows such interrogation if there is an "imminent threat," but was given a Miranda warning once it was determined that there was no ongoing threat. Officials said he waived his rights and continued to cooperate.

Lawmakers on both sides of the aisle suggested that there is a basic hole in the intelligence system that is difficult to fill. "Increasingly, the dilemma is the well-educated man who moves through the education system of our country somewhat promisingly," said Sen. Richard G. Lugar (R-Ind.); Shahzad is a graduate of U.S. universities.

"I've always felt that this was the future in America for what we have to watch for in terrorism," said Sen. John D. Rockefeller IV (D-W.Va.), a member of the intelligence committee. "And it's very hard to protect against, because you don't know who they are."

After an aborted bombing attempt aboard a plane on Christmas Day, President Obama cited a systemic failure that allowed the suspect, a Nigerian citizen, to travel on a commercial airliner despite intelligence warnings about his possible connection to terrorism -- some of which came from his parents -- and allegedly with a bomb in his underwear. Like the Times Square bomb, that explosive malfunctioned.

Obama ordered a major review of watch-list procedures and failures that had also allowed the suspect, Umar Farouk Abdulmutallab, to retain a valid U.S. visa. Officials said Tuesday that the criteria for searching and questioning individuals had been tightened since then but that the administration is still discussing whether to alter its standard of "reasonable suspicion" of terrorism connections.

Even if new criteria had been in place, however, officials agreed that it was unlikely that questions would have been raised over Shahzad's apparently taking several trips to Pakistan, where his parents and other family members lived. Naturalized in April 2009, he lived with his family in Connecticut and until last year was an employed homeowner. He broke no laws and did nothing to call attention to himself.

It was not until the day after the car bomb was found that federal officials discovered a foreign connection, linking a telephone number used by the purchaser of the vehicle to Pakistan and, by early Monday morning, to Shahzad. They gleaned details about him from an unrelated airport screening conducted when he returned from Pakistan in February, a requirement for all passengers from certain countries instituted after the Christmas Day attempt. The requirement was eliminated this spring after the targeted countries, including Pakistan, complained.

At 12:30 p.m. Monday, Shahzad's name was added to the terrorism suspect database at the National Counterterrorism Center and airlines were alerted through a "Web board" -- a secure government site with an updated list of passengers who should be blocked from flying.

By 4:45 p.m., Shahzad's passport number was added to the Web site, officials said. The government and Emirates airline pointed fingers at each other over what happened next. The company said that when the list was updated, Shahzad, who booked the flight at 6:30 while traveling to the airport, was not yet on the manifest. Emirates officials said they informed the government that a last-minute, one-way ticket had been reserved and paid for in cash, but got no response. Officials said they had no record of that message.

"If he was put on the no-fly list before he arrived at the airport, then he never should have been allowed to board the plane in the first place," said Sen. Susan Collins (Maine), the ranking Republican on the Homeland Security Committee. She added, however, "It's evident to me, in contrast to the Abdulmutallab case, there was much better coordination this time at the federal level between our intelligence agencies and our law enforcement agencies."

It was unclear exactly when Shahzad was first physically located. "The situation was extremely fast-moving," an official said. "Within a few hours," he said, the FBI had "begun surveillance. At some point, he was able to slip surveillance."

Another official said it was possible that the FBI was tracking more than one person in similar vehicles, without knowing for sure which person was Shahzad or where he was going.

At 10:40 p.m., 20 minutes before takeoff, the airline sent a "last look" passenger manifest to Customs and Border Protection, and the suspect hit "as a possible match," an official said. Agents reached the plane just after the door had been closed.